One known method for non-contacting measurement of positions and various measures, such as height and width, of tubes, rods, beams or like products is to use optical methods based on shading or reflection of beams, or by processing of images taken by a video camera.
The environment in many manufacturing processes, involving, above all, pollution and often high temperatures, deteriorates the reliability and the accuracy of such equipment.
It is previously known to measure the measures and position of an electrically conductive measuring object by means of inductive methods. In that case, a transmitter coil is used which generates a time-varying magnetic field which induces currents in the conductive measuring object. These currents generate a magnetic field which, in turn, induces a voltage in a receiver coil, this voltage being dependent, among other things on the shape, the conductivity, and the magnetic permeability of the measuring object, as well as on the geometrical conditions. From this voltage, under certain conditions, geometrical dimensions, such as distances and the position of the measuring object, may be calculated. For generating a time-varying magnetic field, sinusoidal currents in the transmitter coil may be used, as described in U.S. Pat. No. 4,475,083, or a constant current which is suddenly interrupted, such as described in U.S. Pat. No. 5,059,902, may be used. The latter method is more robust from the point of view of a measurement technique and facilitates the separation of different properties of the measuring object. One problem with these measurement devices, however, is to determine the measures of the measuring object when its position is changed.
U.S. Pat. No. 5,270,646 discloses a method of arranging coils so as to achieve measurement of the width of a strip. However, the technique can only be used for a strip of relatively limited width. Further, it is assumed for correct function that the edge of the strip is substantially plane. For many applications, the accuracy is not sufficient, primarily when there are large distances between the strip and the measuring coils, which is due to difficulties in correctly compensating for variations in the distance.
Common to prior art devices for inductive measurement of distance, thickness and measures of electrically conductive objects, derived therefrom, is that the transmitter and receiver coils are arranged with the same symmetry axis or are located on different sides of the measuring object. It also occurs that the same coil is used as transmitter and receiver coil. The magnetic field generated by the transmitter coil then becomes substantially perpendicular to the surface of the measuring object at the measuring point, or at least has a large component towards the surface of the measuring object. This results in currents and magnetic fields from different depths into the measuring object contributing to the measurement signal which thus becomes both material-dependent and dependent on the thickness and shape of the measuring object in a relatively large region around the location where measurement is to take place.